1. Field of the Invention
This invention relates to a polymer-dispersed liquid crystal display device and a method for production thereof.
2. Description of the Related Art
Liquid crystal display devices and plasma emitting display devices are attracting attentions as flat panel display devices which are lighter and smaller than the CRT display device. These display devices can be roughly divided into two types, namely a self-emissive type which emits light on its own to display and a non-emissive type, or transmission controlling type, which has a separate light source and controls the transmission of light to display.
A typical one among the transmission controlling type display devices is a liquid crystal display device which is becoming commercially practical extensively in various fields as the most likely next generation of the display devices.
There will be described a fundamental concept of a projection display apparatus.
FIG. 13 shows the arrangement of the projection display apparatus. The projection display apparatus comprises an optical system having a spheroidal mirror 101, a light source lamp 102, a collimator lens 103, an electric aperture stop 104, a scattering type liquid crystal panel 105, a field lens 106, projection lenses 107, and another electric aperture stop 108. The light from the lamp 102 enters the collimator lens 103 through the electric aperture stop 104 directly and after being reflected by the mirror 101. The collimator lens 103 outputs this incident light as parallel rays to the scattering type liquid crystal panel 105. The scattering type liquid crystal panel 105 includes a liquid crystal layer, in which a liquid crystal material is dispersed in a polymer resin, as a light-modulating layer between a pair of transparent electrode substrates. The scattering type liquid crystal panel 105 is driven by a modulating device driver (not shown) as a light-modulating device which modulates the spatial propagation direction of light by using this light-modulating layer. The modulated light from the scattering type liquid crystal panel 105 is incident on the projection lenses 107 with the electric aperture stop 108 through the field lens 106. The projection lenses 107 project the modulated light onto a reflection screen 109. That is, the basic display concept of this projection display apparatus.
The projection liquid crystal display device is known as a display device for displaying a large size image. This device collects light from a lamp onto a liquid crystal display panel and projects the transmitted light or reflected light on a screen through a projection lens to display an image in a large size.
The liquid crystal display panel can two-dimensionally modulate a light intensity, so that the transmitted light is modulated, and the modulated light is magnified and projected through an optical system to realize a projection image in a large screen size.
Thus, the liquid crystal display devices are known in various types. Most of them adopt the principle of operation represented by the previously commercialized twisted nematic liquid crystal, which principle uses the birefringence or rotatory polarization of light in a liquid crystal layer and the linear polarization of a polarizer to control the light coming out on the side of the observation face of the liquid crystal display panel, thereby displaying.
But, the liquid crystal display device using the birefringence or rotatory polarization of light in a liquid crystal layer and the line polarization of a polarizer has a disadvantage that about a half of light intensity from the light source is lost in the polarizer in view of the principle of operation. Namely, the efficiency of using the light from the light source is low because the light intensity lost in the polarizer does not contribute to displaying. As a result, there are disadvantages that the screen luminance is low, and a powerful light source is required to display at high luminance, increasing power consumption and generating heat.
On the other hand, the liquid crystal display device controlling a direction (or scattering/transmission) of outgoing light beam does not need a polarizer, improving the efficiency of using the light from the light source.
A polymer-dispersed liquid crystal display device, which is promising as the liquid crystal display device controlling a direction of outgoing light beam to display, combines a polymer synthetic resin and liquid crystal as separate phases. More specifically, combination does not dissolve one of them into the other to form one body, but disperses and holds one of them into the other. For example, a liquid crystal composition such as TN (twisted nematic) liquid crystal is dispersed and held as a liquid crystal phase in continuous or many capsule-shaped cavities in a polymer resin phase.
The above polymer-dispersed liquid crystal display device is believed to be particularly promising among the liquid crystal display devices which improve the efficiency of using the light from the light source. And, a polymer-dispersed liquid crystal layer is filled between two transparent electrodes which are disposed to face each other with a space therebetween.
In the polymer-dispersed liquid crystal display device, the polymer-dispersed liquid crystal scatters the light transmitting through it when a voltage is not applied between the two transparent electrodes. Therefore, the display is observed to be translucent white. On the other hand, when a liquid crystal drive voltage is applied between the two transparent electrodes, the polymer-dispersed liquid crystal layer becomes transparent according to the voltage applied and the light is not scattered, so that the display is observed to be highly luminance. Thus, the polymer-dispersed liquid crystal display device is a display device which controls the scattering/non-scattering of light. Combination of this display device with an optical system which extracts scattered light only or non-scattered light only can convert the control of scattered light to the control of light intensity (luminance on the display).
Based on the above operation principle, a liquid crystal display device, which does not require a polarizer, has a high efficiency of using light from a light source and can display an image with high luminance, must be realized.
Though the polymer-dispersed liquid crystal display device is expected to have the above remarkable characteristics and can display a high luminance and high definition image, only a few models have been put to use. This is because conventional production technologies are not adequate and there are various technical problems which cannot be solved easily. One of the conventional major problems is nonuniformity.
Major causes of the nonuniformity include an uneven thickness of a liquid crystal layer (or uneven liquid crystal layer thickness) in addition to variations in material properties of a liquid crystal composition.
When the liquid crystal layer thickness is not uniform in a display region, a field strength in the liquid crystal layer is variable even when a constant voltage is applied. In addition, an optical path length, which contributes to the scattering, is varied. These causes induce uneven luminance (brightness) in displayed images, which is recognized as a nonuniformity in the screen.
Now, the correlation between a direction of change in display and a variation in thickness of a liquid crystal layer will be considered. First, a conventionally used TN mode liquid crystal will be considered. The TN mode liquid crystal is operated at a certain voltage without depending on the liquid crystal layer thickness because of the properties of liquid crystal material as indicated by the expression (5.36) on page 331 of Liquid Crystal Device Handbook (No. 142 Committee of Japan Industry Association). It is obvious from the following expression seen in the above Liquid Crystal Device Handbook: EQU Vth=.pi.[{K11+(K33-2K22)/4}/.epsilon.0..DELTA..epsilon.].sup.1/2.
But, the polymer-dispersed liquid crystal has nematic liquid crystal dispersed in a polymer resin phase, so that a voltage is distributed between minute liquid crystal drops (generally diagrammatized as a so-called capsule liquid crystal phase or a network like liquid crystal phase) and polymer resin (generally diagrammatized as a so-called matrix polymer resin phase), resulting in that the behavior of liquid crystal molecules depends on an electric field.
The thicker the liquid crystal layer increases, the weaker the electric field strength becomes. More specifically, assuming that an electric field strength is E, a liquid crystal layer thickness is D and an applied voltage is V, the electric field strength is expressed as E=V/D. And, the electric field strength E is inversely proportional to the liquid crystal layer thickness D. Therefore, variations (dimensional variation) in the liquid crystal layer thickness cause variations in the electric field strength of the liquid crystal layer in the display region, resulting in nonuniformity in projected pictures.
A conventional liquid crystal display device disperses many spherical- or bar-shaped fine spacers made of glass or rigid plastic having a uniform diameter into a liquid crystal layer thickness of liquid crystal cells and a sealant for sealing its periphery including a case of the polymer-dispersed liquid crystal, and holds them between two opposed substrates to keep a space between the two substrates, thereby making the liquid crystal layer uniform. And, because of its operational theory, the polymer-dispersed liquid crystal display device needs to make the liquid crystal layer thicker than in an ordinary TN liquid crystal display device to increase a light scattering ability of the liquid crystal layer in order to realize a high contrast property.
However, when the liquid crystal layer is thick or the pixel size is very small, the spacers have a relatively large diameter with respect to the pixels, so that the variations in the spacer diameters and the ununiform liquid crystal layer thickness have noticeable adverse effects on a display made of pixels. Therefore, the spacers' arrangement density and position are tightly limited. As a result, a small number of spacers is required to be positioned in a limited area, making it difficult to uniformly control the thickness of the liquid crystal layer in the display surface. Namely, there is a disadvantage that the liquid crystal layer thickness does not have proper accuracy.
In the projection type liquid crystal display device, the pixel size is very small as compared with a direct-view type liquid crystal display device, the spacers are required to be dispersed in quite a low density or to be arranged while avoiding pixels. Besides, to be used for the projection type liquid crystal display device, the polymer-dispersed liquid crystal cell is required to be thick as described above, making it difficult to provide a uniform liquid crystal layer.
In the polymer-dispersed liquid crystal display device, the display condition of the liquid crystal layer are depended on the electric field as described above. In particular, the nonuniformity is noticeable when a drive voltage is high.
Thus, the conventional polymer-dispersed liquid crystal display device has a problem of producing the nonuniformity due to the variations in the thickness of the liquid crystal layer in the display region. It also has a problem that the nonuniformity becomes noticeable depending on the drive voltage.
This invention has been completed to remedy the above problems, and aims to provide a polymer-dispersed liquid crystal display device which does not suffer from the nonuniformity and realizes a uniform display in the projected image.